Color and Perceptual Variation Revisited: Unknown Facts, Alien Modalities, and Perfect Psychosemantics Jonathan Cohen∗ Optimism, said Candide, is a mania for maintaining that all is well when things are going badly. — Voltaire, Candide, ch. 19. Abstract An adequate ontology of color must face the empirical facts about per- ceptual variation. In this paper I begin by reviewing a range of data about perceptual variation, and showing how they tell against color physicalism and motivate color relationalism. Next I consider a series of objections to the argument from perceptual variation, and argue that they are un- persuasive. My conclusion will be that the argument remains a powerful obstacle for color physicalism, and a powerful reason to believe in color relationalism instead. Suppose that colors are real rather than illusory properties of objects. Then what sorts of properties are they? Two competing views that have attracted philosophical adherents are the following: Color Physicalism Colors are mind-independent, circumstance-independent (typically, physical) features of their bearers. They are, in this respect, analogous to shape properties such as being square.1 Color Relationalism Colors are relational properties — properties consti- tuted in terms of a relation between their bearers and subjects (possibly inter alia). They are, in this respect, analogous to a property such as being a sister. ∗Department of Philosophy, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0119, [email protected] 1‘Physicalism’ is a curious label for this view, especially in the context of the contrast with relationalism. For (i) relationalism also allows that colors are physical (as do, for that matter, many other views that self-described color physicalists reject), and (ii) the view presently under consideration comes without any substantive characterization of the physical. That said, I’ll adhere to the label preferred by the defenders of the view for present purposes. 1 One of the most important motivations that has been used to argue against color physicalism and in favor of color relationalism arises from a set of em- pirically motivated considerations about perceptual variation. The thought, roughly, is that, if color physicalism is right, then in every case in which a given stimulus produces distinct effects on different perceptual systems that count as representing that stimulus, at most one of those effects can count as a veridical representation; but it has been thought that, in a wide variety of cases, this en- tailment of the view is deeply implausible. In contrast, color relationalism makes no such entailment, but instead allows for an attractive ecumenicism regarding perceptual variation. The conclusion one is invited to accept, then, is that the phenomenon of perceptual variation favors relationalism over physicalism. This general form of argument, which I’ll call ‘the argument from perceptual variation’ is not novel (it can be found in some form in the writings of Galileo, Hume, and Locke). But it has been pressed anew in recent papers including Block (1999), Cohen (2006a), Cohen (2003b), Cohen (2006b), Cohen (2006c), Hardin (1988), Jakab and McLaughlin (2003), and McLaughlin (2003b); unsur- prisingly, the authors of all these papers are hostile to color physicalism, and several of them (but not all) are sympathetic to color relationalism. Predictably, however, defenders of physicalism have recently raised a number of objections to the argument from perceptual variation in an attempt to rebut the challenges that that argument poses for their view. This paper is an attempt to buttress the argument from perceptual variation against these objections. It is not a general defense of color relationalism (for that, see Cohen (2006a)), but a local attempt to block objections to the argumentative strategy that, in my view, most strongly motivates color relationalism. In §1 I’ll review the argument from perceptual variation by considering several of its instances, and I’ll show how the argument raises difficulties for color physicalism. Next, in §2 I’ll show how abandoning color physicalism in favor of color relationalism resolves these difficulties in a simple and consistent way. Then, in §§3–5 I’ll consider and respond to a number of objections to the argument. My conclusion will be that the argument remains a powerful obstacle for color physicalism, and a powerful reason to believe in color relationalism instead. 1 Physicalism and Perceptual Variation The leading current form of physicalism maintains that (surface) colors are classes of surface spectral reflectance distributions.2 2A surface will reflect some percentage of the light of wavelength λ that falls on it. If we collect the percentages of reflected to incident light for each visible wavelength, we will get a function (from visible wavelengths to numbers in the interval [0,1]) that characterizes the disposition of a surface to affect light in the visible range in a certain way. This function is the surface’s spectral reflectance distribution. Physicalists typically prefer to identify colors with classes of such functions (rather than with the functions themselves) because of the phenomenon of metamerism: under any given illumination, an infinite number of surfaces (distinct in their surface spectral reflectance dis- tributions) will be visually indistinguishable for a given observer. What this suggests is that 2 The challenge raised by perceptual variation for this view comes from the empirical observation that a given stimulus produces a remarkably wide vari- ety of effects in different perceptual systems, and produces a remarkably wide variety of effects on a single perceptual system when viewed under different per- ceptual circumstances. The difficulty is that, on standard assumptions, each of these different effects is a representation of the color of the stimulus. But if col- ors are mind-independent and circumstance-independent properties of surfaces, as are spectral reflectance distributions (or classes thereof), then physicalists are committed to saying that at most one of these varying effects represents the color of the stimulus veridically. However, the objection goes, it is hard to see that anything could (metaphysically) make it the case that one of the variants is veridical at the expense of the others: it seems that any considerations that could be brought forward in support of the veridicality of one of the variants could be matched by considerations of equal force in favor of some other variant. Significantly, the sort of variation of color vision at issue is no mere imagined possibility: there is overwhelming and unambiguous evidence of actual varia- tion of color vision. In particular, I want to focus attention on actual examples of three types of variation: variation between subjects of different species, in- terpersonal variation between subjects of our own species, and intrapersonal variation in a single human visual system.3 First consider the following example of interspecies variation. The pigeon on the window ledge has a tetrachromatic visual system: an arbitrary color stimulus can be perceptually matched for a pigeon by a linear combination of four appropriately chosen primaries. In contrast, normal human visual systems are trichromatic: an arbitrary color stimulus can be perceptually matched for a human being by a linear combination of three appropriately chosen primaries. Consequently, there are pairs of surfaces that are perceptual matches for human visual systems but not for pigeon visual systems. This entails that there is a difference between the way at least one surface of the pair looks to pigeon visual systems and the way it looks to human visual systems. That is, the pigeon visual system represents the surface in question in a way that is psychophysically distinguishable (viz., in terms of whether it matches the other surface of the pair) from the way in which the human visual system represents the same surface. There is also substantial interpersonal variation in color vision between (nor- mal trichromatic) human visual systems. Perhaps the most discussed instance of this sort of interpersonal variation, discussed at length in Hardin (2004), is the variation in the spectral wavelength (alternatively, in the Munsell chip) se- identifying colors with reflectance functions yields an excessively fine-grained individuation of the colors. The move to identify colors with classes of reflectance functions is intended to get around this problem. (An exception to this generalization is Churchland (2007); Churchland identifies colors with reflectance functions, and is prepared to live with the resultant extremely fine-grained individuation of colors.) 3For brevity I provide here only a single instance of each type of variation. Since this has the disadvantage of underplaying the scope and seriousness of the problem, and also the importance of a truly general response to the cases, I invite the interested reader to consult the more extensive discussions of variation in Cohen (2006a), Thompson (1995), and Hardin (1988). 3 lected by subjects as unique green (i.e., as looking greenish without looking at all bluish or at all yellowish). When two normal trichromatic observers view chip C under identical perceptual conditions, C looks unique green to one of them but bluish green (hence not unique green) to the other. Once again, the way one subject represents the color of C is psychophysically distinguishable (viz., in terms of whether it is represented